This invention relates to a novel method and structure for producing bumps for IC package substrate, and more particularly to a Flip-Chip BGA substrate with solder bumps form thereon.
Nowadays, many companies are trying very hard to approach smaller and more effective IC chips technology of Integrated Circuit Chip. In addition to the improvements of IC chips, the IC Package Technology also has great achievement. From Lead Frame Package to BGA Package and to Tape Automatic Bonding BGA, TAB BGA, kinds of IC Package technologies are invented for IC chips, which are smaller and smaller again. To adapt for advance technology, mini BGA and Chip Scale Package (CSP) are created for the scale of substrate/chip value less than 1.2. And to do so, most of them apply Flip-Chip technology to package these chips with package substrates.
Please refer to FIG. 1; a prior Flip-Chip technology is present in it. The prior Flip-Chip technology first applies a BGA substrate 11 and forms a circuit layer 12 and a protective layer 13 thereon. Several contact windows are opened to expose some portions of the circuit layer 12. A solder pad 14, which is preferably made up by copper, tin and lead materials, is electroplated on the exposed circuit layer 12 in the contact window and a bump 15 is thereafter formed on.
On active side of the IC chip 21, an Aluminum alloy layer 22 and a protective layer 23 are formed thereon. A solder bump 24 (or gold bump) is also formed on the IC chip 21, which is contacted with the Aluminum alloy layer 22. During package, the IC chip 21 is heated and pressed onto the BGA substrate 11 to merge the solder bump 24 and the bump 15, by which not only electrically connects the IC chip 21 and the BGA substrate 11 and also binds themselves into one complement.
However, such a structure has some disadvantages as described hereunder.
1, the pitch between the bumps, which are made by screen print technology, are mostly around 200xcx9c250 xcexcm. By screen print technology or attaching BGA bumps onto the substrate, which are not very critical techniques, it is very difficult to make pitches less than 150 xcexcm.
2, the cost is higher to make bumps 24 on the IC chips 21. The facilities for producing IC are very expensive. In addition, the integrated circuits, which are composed by light doped silicon, silicide and metal Aluminum, are very complicated and lower heat-resistant. The bumps on the IC chip are no doubt made under critical conditions by costly facilities. Thus, the cost is higher and the yield is lower. On the other side, circuits on the BGA substrate are made by copper, which can endure higher heat, and are much easier to make than those on IC chip. So, if bumps were made on the BGA substrate, those disadvantages can be overcome.
The object of present invention is to form bumps on the Flip-Chip BGA substrates. This object no longer forms the bumps formed on IC chips but BGA substrates. The bumps on the BGA substrates can still work for IC chips soldering therefore to reduce cost.
The other object of present invention is to replace bump screen print technique by metal electroplating to scale down the distance between bumps, even less than 150 xcexcm. So the feature size of the package device could be further minified.
Therefore, the embodiment of present invention for producing bumps on BGA substrate comprises the steps of:
A) Applying a substrate, on which a circuit layer in formed thereon. A protective layer is formed on the circuit layer and there are several contact windows opened therein to expose the circuit layer.
A1) Depositing a medium layer on the protective layer, which is also opened at the contact windows.
B) Electroless metalizing a metal layer on the surface of the substrate, which covering the surface of the medium layer, the inside surface of the contact window, and the surface of the exposed circuit layer.
C) Forming a dry film on the metal layer, which is opened at the contact windows to expose the metal layer.
D) Forming metal pads with a predetermined thickness on the exposed metal layer.
E) Forming bumps on the metal pads.
F) Removing the dry film and the metal layer thereon which protrudes the bumps out.
G) Reflowing and flatting the bumps to generalize them into standard scale.
In this invention, protruded bumps are formed on the BGA substrate for soldering with an IC chip. Therefore, the IC chip no longer needs to make bumps thereon. And the electroplating technique is used as a replacement of the screen print technique to scale the distance between bumps down to 150 xcexcm and around 80xcx9c120 xcexcm. Therefore, the improved BGA may be applied on to smaller IC chips to catch up the trend.
The structure of this invention comprises a substrate, a circuit layer made on the substrate, a protective layer on the circuit layer with contact windows therein, a metal layer on the circuit layer, metal pads on the metal layer, and bumps on the metal pads for soldering with an IC chip.
However, some solder balls are also formed on the other side of the substrate for connecting with a motherboard. For connecting with a motherboard, the pitches of these solder balls are more than 200 xcexcm, which have different purposes and producing method with present invention.